This invention relates to a voltage to frequency converter and more particularly, to a converter which generates a pulsed output signal having a frequency and duty cycle corresponding to the voltage and polarity, respectively, of an applied input signal.
A voltage to frequency converter is a circuit which receives an input signal and generates a periodic output signal having a frequency that varies in accordance with the voltage of the input signal. Typically, the output signal from the converter is in the form of a series of logic level pulses, with the pulse rate of the output signal varying according to input signal voltage.
Such converters have many applications. For example, an unknown analog signal may be applied to the input of a voltage to frequency converter and the output pulses therefrom may be accumulated in a digital counter for a fixed period of time. The digital number contained in the counter can then be displayed to provide a visual indication of the input signal voltage. In many applications, it is required that the voltage to frequency conversion take place with a high degree of accuracy.
A typical prior voltage to frequency converter includes an input signal integrator comprising a capacitor in the feedback loop of an amplifier. The input signal is applied to the integrator through an input resistor. The capacitor is then charged at a rate determined by the voltage of the input signal and by the product of the capacitance of the capacitor and the resistance of the input resistor. When the output of the integrator reaches a predetermined level, the input signal is removed and a reference signal is applied until the capacitor is completely discharged. This sequence is typically repeated so that the converter oscillates at a frequency determined by the time required to charge and discharge the capacitor. The voltage of the input signal is thus proportional to the frequency of this oscillation.
The accuracy of a voltage to frequency converter of this type depends critically upon the stability of the capacitor and input resistor used therein. If the characteristics of these components change, or drift, with time, the frequency at which the converter oscillates changes even if the input signal voltage remains constant. Errors thus develop in the voltage to frequency conversion. To insure relatively high accuracy, the capacitor and input resistor used in these converters must typically be precision, stable components. Such components, however, are relatively expensive and add to the overall cost of the converter.
Another prior voltage to frequency converter utilizes an input signal reversing technique. The input signal is applied to an integrator, and the polarity of the input signal is reversed each time the integrator output reaches a predetermined level. Specifically, the input signal is applied to the integrator through a polarity reversing switch. A feedback capacitor in the integrator is alternately charged and discharged by the same voltage, but of opposite polarity. The average current through the input resistor and the average voltage across the capacitor are thus zero. Errors due to changes in the characteristics of these components thus tend to cancel with time.
In a voltage to frequency converter of that type, however, the accuracy of the output is dependent upon the stability of the input signal polarity reversing switch. If the switch resistance drifts, for example, errors develop. Also, when the input signal varies rapidly in polarity, due, for example, to noise near the zero voltage point, the polarity switch often gets confused, causing the input signal balancing cycle to be upset and errors to develop.
Still another prior voltage to frequency converter applies an input signal through an input resistor to a summing point which also receives either a positive or negative reference signal. The output of the summing point connects to the input of an integrator. The polarity of the reference signal that is applied to the summing point is selected by comparing the output of the integrator with certain fixed threshold voltages. The comparison is performed by two separate logic components, one for a positive input signal, and the other for a negative input signal.
This converter does not require an input signal polarity reversing switch, and thus eliminates the errors that can result therefrom. It, however, requires two separate switches and two separate logic components for controlling the application of the positive and negative reference signals to the summing point. The characteristics of these two switches and components can vary relative to one another, causing the applied reference signals to vary relative to one another in magnitude and duration. This can effect the accuracy of the converter. Also, in this converter, the average current passing through the input resistor is equal to the input voltage divided by the resistance of the input resistor. Thus, changes in the resistance of the input resistor directly affect the accuracy of this converter. To achieve high accuracy, this converter also requires a precision, stable input resistor.
Accordingly, it is a general object of the present invention to provide an improved voltage to frequency converter.
Another object of the invention is to provide a voltage to frequency converter which is relatively accurate but which does not require precision components.
Still another object of the invention is to provide a voltage to frequency converter which is relatively insensitive to noise in the input signal, particularly, noise around the zero voltage point.
SUMMARY OF THE INVENTION
In accordance with the present invention, a voltage to frequency converter receives an input signal of unknown voltage and polarity and generates a pulsed output having a frequency and duty cycle corresponding to the voltage and polarity, respectively, of the input signal. The input signal and a reference signal of predetermined magnitude and polarity are introduced to an integrator which provides an output that ramps in a positive or negative direction depending upon the polarity of the reference signal applied. A level detector determines whether the output of the integrator is greater or less than a predetermined level. A clock signal controlled logic section connects to the output of the level detector and controls a reference signal switch. The reference signal switch applies either a positive or negative reference signal to the integrator depending upon the logic level of the output of the logic section.
The logic output continuously shifts between first and second logic levels causing the reference signal switch to apply positive and negative reference signals to the integrator alternately. As a result, positive integration cycles are continuously opposed by negative integration cycles. Each integration cycle also has a duration which is an integral multiple of a clock signal pulse length.
The output of the converter is taken from the output of the logic section. This output has a zero frequency when the input signal magnitude is equal to that of the positive or negative reference signal, one half the frequency of the clock signal when the input signal magnitude is halfway between the positive and negative reference signals (i.e., at zero volts), and a proportional frequency when the input siganl magnitude is between these two extremes.
Also, with the input signal at zero volts, the duty cycle of the pulsed output of the logic means is exactly 50 percent. A positive polarity of the input signal causes the output of the logic to have a duty cycle greater than 50 percent, while a negative input signal causes the logic output to have a duty cycle less than 50 percent. Changes in the voltage and polarity of the input signal are directly reflected as changes in the frequency and duty cycle of the logic output.
Because positive and negative integration cycles are continuously alternated in the converter, errors caused by variations in the frequency and pulse symmetry of the clock signal tend to cancel. Additionally, errors caused by changes in the characteristics of the resistive and capacitive components used to determine each integration cycle also tend to cancel. As a result expensive, precision components are not required. The converter is thus relatively economical, yet provides a highly accurate indication of the magnitude and polarity of the input signal.
The invention is pointed out with particularity in the appended claims. The foregoing and other features and advantages of the invention will be better understood from the following detailed description taken in conjunction with the accompanying drawings.